VII. QUICK-FREEZING AND FREEZING-DRYING 221 



ing the containers and other manipulations to achieve either rapid 

 or slow transfer of heat depending upon which type of freezing is de- 

 sired. 



2. Apparatus for Dehydrating Frozen Material 



Many of these have been described but all operate basically on 

 similar principles. Either low temperature condensation or chemical 

 removal of water vapor under high vacuum is used for laboratory type 

 equipment. Typical apparatus is illustrated in Figures 1 and 2. A 

 full bibliography of the various published types of apparatus will be 

 found elsewhere (2). Packer and Scott (46) have reported on a sim- 

 plified cryostat for the dehydration of frozen tissues. This was an al- 

 most entirely glass apparatus permitting a dehydration temperature 

 for the product as low as — 70°C. They claim low cost of operation; 

 Dry Ice was used for the refrigerant. Suitable lyophile equipment 

 (24) utilizing Dry Ice condensation and freezing, and Cryochem type 

 of equipment using a regenerable chemical desiccant (8) have been de- 

 scribed. Detailed consideration of various principles employed in 

 applying these methods to medical products was published in 1945 

 (25). Laboratory apparatus is available commercially in a wide 

 variety of sizes and types (48) . 



E. TECHNICAL OPERATIONS AND LIMITATIONS 



1. Criteria for Quality of Preservation 



As far as is known, many frozen-dried preparations such as blood 

 plasma, penicillin, antitoxins, convalescent serum, some species of 

 microorganisms, and other products have virtually an indefinite life. 

 Naturally, time must elapse before the full extent of preservation can 

 be established for any given product. Accelerated testing may be 

 carried out by use of higher temperatures such as the rates of deterio- 

 ration at 20, 50, and 100°C. Higher temperature, however, is not 

 always an accurate index of what may be expected at ordinary tem- 

 peratures. There may be a critical temperature level above which 

 a given product will be almost instantly destroyed. For this reason, 

 everj^ preparation must be considered individually. 



The original properties of some materials may be destroyed in the 

 process. For example, multicellular organisms are killed. Spiro- 

 chetes and similar organisms do not survive freeze-drying. In pre- 



